1. Field of the Invention
The invention relates to a proton conduction material that can be suitably employed in a fuel cell, a unit for electrolyzing water, a hydrogen sensor, a concentration cell, a dehumidifier or the like, and to a method of manufacturing such a proton conduction material. More specifically, the invention relates to a proton conduction material that can be suitably employed in a catalytic layer of an electrode of a fuel cell, and to a method of manufacturing such a proton conduction material.
2. Description of the Related Art
A solid polymer type fuel cell has an electrolytic membrane and a catalytic layer provided on both faces of the electrolytic membrane. In general, an electrolytic membrane and a catalytic layer are constructed as a membrane-electrode assembly (MEA).
The electrolytic membrane is made of a proton conduction material which smoothly conducts protons produced in reactions occurring in a cell and which exhibits low electrical conductivity. In most cases, polymer materials of perfluorocarbon sulfonic acid type typified by Nafion® (manufactured by Du Pont Co., Ltd.) are used as proton conduction materials that are practically useful in the field of solid polymer type fuel cells.
As a proton conduction material other than the polymer materials of perfluoro sulfonic acid type such as Nafion® or the like, a polymer material which is reinforced by metalloxane (including siloxane bonds) and which has a sulfonic acid functional group is cited as the related art in Japanese Patent Application Laid-Open 2001-11219 and Published Japanese Translation of PCT Application 10-510090.
In general, a catalytic layer has a structure in which catalytic particles functioning as a catalyst are dispersed in a proton conduction material. In addition to proton conductivity, an electrolytic membrane is required to have the ability to prevent reactive gases in both electrodes from crossing over to each other, namely, gas sealing properties. On the other hand, it is desired that the proton conduction material contained in the catalytic layer exhibit high gas permeability to allow the reactive gases to be supplied to catalytic activation points of the catalytic particles covered with the proton conduction material. If the proton conduction material contained in the catalytic layer exhibits low reactive gas permeability, formation of reactive points (three-phase interfaces) is difficult.
Nafion® exhibits high reactive gas permeability in the presence of water content. Therefore, in order to control reactive gas permeability, the percentage of water content needs to be adjusted. Too high a percentage of water content also may cause inconveniences such as flooding and the like. Not to mention Nafion®, any hygroscopic proton conduction material causes liquid water absorbed therein to dissolve reactive gases and allows the reactive gases to permeate therethrough. Proton conduction materials in a wet state seldom raise a problem of the deficiency of reactive gas permeability.
However, if a proton conduction material contained in a catalytic layer is dry during a starting operation of a fuel cell or the like, the amount of water as a medium for displacing reactive gases is insufficient, so that a sufficient amount of the reactive gases cannot be supplied to the catalytic activation points of catalytic particles. Therefore, if the catalytic layer is not wet enough, the fuel cell cannot perform its function properly. Similarly, since the percentage of water contained in the proton conduction material in the catalytic layer cannot be controlled appropriately under conditions such as non-humidified operation and the like, the fuel cell cannot perform its function properly.